This invention relates to fluoropolymers and more particularly to dihydroxy-terminated poly(2,2,3,3,4,4-hexafluoropentane-1,5-diol formal).
The copolymerization of diols with formaldehyde has in the past been accomplished by a number of means. Originally, the most common method employed was acid transformylation of the diol with a dialkyl formal. Usually, the acid catalyst was p-toluenesulfonic acid (see J. W. Hill and W. H. Carother Amer. Chem. Soc., 57, 925 (1935); F. D. Trischler and J. Hollander, J Polym. Sci. A-1, 5, 2343 (1967)). This procedure, however, required high temperatures and constant removal of the alcohol byproduct in order to drive the reaction forward to good yield.
Subsequently, formation of polyformals has been achieved by reaction of the diol with formaldehyde either in the form of paraformaldehyde or in the form of trioxane. The procedures also called for reactions to take place at high temperatures, in a water immiscible solvent, and in the presence of an acid catalyst. Water byproduct formed during the polycondensation process was also required to be removed by azeotropic distillation in order to drive the reaction forward to good yield. (See W. J. Jackson, Jr., and J. R. Caldwell, ACS Adv. Chem Ser. 34, 200 (1982); F. D. Trischler and J. Hollander, J. Polym. Sci. A-1, 5, 2343 (1967)).
Alternatively, synthesis of polyformals has been carried out by acid catalyzed ring opening polymerization of the preformed cyclic formal. This method, by its nature, obviates the need for high temperature reactions and continuous removal of condensation products since none exist. (See J. Furukawa and K. Tada in "Kinetics and Mechanisms of Polymerization", Vol. 2, K Frisch & S. Reegen eds., M. Dekker, New York, 1969, pg. 159ff.)
The object polyformal has been previously synthesized using all three of the aforementioned methods. (see F. D. Trischler and J. Hollander, J. Polym. Sci. A-1, 5, 2343 (1967); P. Johncock, Brit. Pat. No. 1294657.) However, all of these procedures suffer from deficiencies which the present invention was designed to overcome. The transformylation procedure requires high temperatures and facility for continuous removal of the alcohol product. There is no molecular weight control, the yields obtained were abysmally low, and the polymer terminal groups were nonfunctional hydrocarbon. The condensation procedure using trioxane also requires high temperature reactions and continuous removal of the condensation byproduct. There is also no molecular weight control with this procedure. In fact, polymers produced using this procedure are merely oligomeric, and are not suitable for curing in many applications. As well, the ultimate conversion (yield) attained for this procedure is well below those obtained using the procedure of the present invention.
The ring opening polymerization technique employed previously to produce the object polyformal overcame the deficiencies presented by the condensation techniques but nonetheless presented others which the present invention was designed to overcome. First, no molecular weight control over the resultant polymer was exercised. Second, sophisticated high vacuum techniques had to be employed in order to introduce small amounts of PF.sub.5 catalyst. In cases where metallic Lewis acid catalysts were employed, high purity solvents free of nucleophilic contaminants had to be prepared for the introduction of precise amounts of catalyst solution. In addition, no precautions were taken to ensure that hydroxyl bifunctionality was retained.